Thienamycin (10 is produced by fungi of Actinomyces and is known to possess extremely potent antimicrobial activities. Its discovery initiated intensive research on carbapenem compounds, resulting in publication of imipenem ##STR3##
However, these carbapenem compounds are far from satisfactory in terms of chemical stability and resistance to renal dehydropeptidase-I (DHP-I). Consequently, investigation has been conducted into more stable carbapenem compounds exhibiting potent antimicrobial activity, which led in 1984 to the finding that 1.beta.-methylcarbapenem (3) having a .beta.-methyl group at the 1-position is stable and possesses strong antimicrobial activity. ##STR4##
The said compound (3) contains four consecutive asymmetric carbons (C.sub.1, C.sub.5, C.sub.6, C.sub.8), and in view of the fact that most of the publications on its synthesis are concerned with the utilization of optically active azetidinone (4) as an intermediate, the synthesis of the intermediate (4) has been considered to be the key to the commercial production of the said compound (3).
According to the literature published so far on the synthesis of the compound (4), four reports dealt with the stereospecific synthesis, one report was concerned with the stereospecific reduction and one report with isomerization, as is illustrated below in the reaction formulas: ##STR5##
These methods suffer from the defects that an increased number of steps are required; that the starting compounds are not easy to be produced; and that the yields are poor.